1. Field of the Invention
This invention relates to the field of spectroscopy, and in particular to a variable spectral resolution agile filter for use in a dispersive spectrometer.
2. Description of Related Art
Spectroscopy is an analytic technique concerned with measurement of the interaction (usually the absorption or emission) of radiant energy with matter. A dispersive spectrometer is a device which measures wavelengths of radiant energy for spectroscopic purposes, and which in general requires a source of radiation, a dispersion device, and a detector.
For many analytic purposes, it is desirable to vary the spectral resolution of the spectrometer, starting the search with a broad spectral resolution and narrowing the spectral resolution when the region of interest is located. For time varying samples, the spectral resolution must be narrowed very quickly, however. Conventional dispersive spectrometers are not suitable for this purpose because, while they can be equipped with variable spectral resolution, the spectral resolution change is slow and is accompanied by a spatial resolution change.
Other known spectrometers are also unsuitable for such spectral analysis of time varying samples. Fourier transform spectrometers, for example, can have variable spectral resolution while preserving spatial imaging, but they require a finite length of time during which the signal is non-varied, or spectral distortions will occur. Spectrometers using Fabry-Perot filters, on the other hand, can have variable center wavelengths and spatial imaging, but are limited to a fixed spectral resolution. Thus, none of the known types of spectrometer achieves both a variable center wavelength and a quickly variable spectral resolution while preserving spatial imaging. In addition, each of the known types of spectrometers, except the Fabry-Perot spectrometer, has moving parts, which can be disadvantageous for many applications, such as for use in long duration missions in outer space.
The present invention solves these problems by providing a quickly adjustable center wavelength without requiring moving parts, using optics similar to those used in the Fabry-Perot spectrometer, while nevertheless also achieving an instantly adjustable spectral resolution. This is accomplished by providing an electrically variable reflective coating on at least one interior surface of an otherwise conventional Fabry-Perot filter.
The use of electrically variable reflectivity is of course known. An example is shown in U.S. Pat. No. 4,923,273 to Taylor, which discloses variable reflectance optic tabs placed in an optical fiber. The optical tabs operate as mirrors in a Fabry-Perot interferometer. However, this device is inherently a one-dimensional device incapable of producing an image, and is intended for use with only a single wavelength. It therefore does not correspond to a variable resolution wavelength filter or spectrometer of the type described and claimed below.